The latest poster children for this controversy are the offspring of Maria del Carmen Bousada. Three years ago, Bousada, a 66-year-old unmarried Spaniard, pretended to be 55 and persuaded a California clinic to impregnate her using donated eggs and sperm. Later that year, she delivered twins. Two weeks ago, she died of cancer at age 69. Her 2-year-old sons are now orphans.

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The case has provoked outcries around the world. "There's a lesson for baby boomers in the story of Maria del Carmen Bousada," wrote columnist Allison Pearson in London's Daily Mail. "The lesson is that, contrary to the fond belief perpetrated by the most self-absorbed generation ever to grace the planet, 60 is not the new 40 and it never will be." Pearson called Bousada "part of an epidemic of older women who think they can get away with defying biological laws that have held good for thousands of years."

That would be a tidy lesson, but it isn't quite true. The biological laws of maternity are shifting. Sixty may not be the new 40, but in some respects, 65 is the new 55. Maternal age is going up in part because, in terms of frailty and longevity, older women aren't as old as they used to be.

The clinic that impregnated Bousada ends eligibility for treatment at age 55. British and Spanish clinics have informal cutoffs at 50. Tony Rutherford, the head of the British Fertility Society, draws the line at 45 in his own practice. A bioethics institute is proposing to enforce the same limit in Spain. Britain's National Health Service cuts off in vitro fertilization at 40.

But what exactly is nature saying? Assisted conception is inherently unnatural. Strictly speaking, nature is telling us not to do it at all. Furthermore, in 1900, life expectancy for a girl born in the United States was 50.7 years. Was nature telling us that women shouldn't live, much less bear children, beyond that point? That didn't stop us from using science to extend women's lives. Why should it stop us from extending their fertility?

For that matter, the British Fertility Society supports IVF, egg donation, and cloning human embryos for research. Why, when the debate turns to menopause, does it suddenly invoke nature?

The second argument for line-drawing is efficacy. Rutherford's clinic cuts off IVF at age 45 because beyond that age, it's unlikely to work. But for many women, egg donation circumvents that problem. So do frozen eggs, embryos, and ovarian tissue. The reason why so many people are freaking out over middle-aged moms is that the old rules of efficacy are gone. These women aren't failing to have babies. They're succeeding.

So the debate boils down to a third argument: These women are too old to raise kids. Why does the clinic that impregnated Bousada draw a line at 55? Because, according to the clinic's owner, any woman who's going to have babies should "survive until the kids reach 18."

A century ago, that would have been a good reason to draw the line at 55. In 1910, an American woman who reached age 55 could expect 18 more years of life. But by 1950, the line had moved: Women could expect 18 more years if they made it age 60. Today, they can expect 18 more years if they make it to age 67. In 2006, when Bousada gave birth at age 66, she had a remaining life expectancy of about 19 years.

Does that mean 66-year-old women should run out and get pregnant? No way. To begin with, the hormone injections that facilitate pregnancy at such an age can be dangerous. In Bousada's case, they may have contributed to the cancer that killed her. Second, life expectancy is an average, not a guarantee. Bousada thought she'd live a long time because her mother made it to 101. Three years later, Bousada was dead. That's the cruelty of averages: Nineteen years of life expectancy means that one 66-year-old woman will live 35 more years, while another will live 3. By definition, the chance that you'll die before your projected life expectancy is 50 percent.

But if health and activity are what's important, why focus on age? Fertility clinics already screen patients for health risks such as obesity, diabetes, and hypertension. No law of biology says a middle-aged woman can't meet such tests. Mary Shearing, who had twins at 53 in 1992, is thriving at 70 today. So are her 16-year-old daughters. Maybe Shearing owes her good health to her daily gym workouts, or maybe she owes both to lucky genes. Either way, should her clinic have turned her away? Is she slower and weaker than most younger moms would be? Sure. But she's also wiser and steadier. Barack Obama was raised by a grandmother in her 50s, and I don't see many folks complaining about how he turned out.

And what about men? Strom Thurmond, the late senator from South Carolina, had kids at 68, 69, 70, and 73. Where was all the "nature" talk then? Do men get a pass if they have younger wives? Shearing's husband was 32 when her kids were born. Shouldn't that count?

Waiting till you're 45 or 50 to bear a child is a bad idea. Waiting till you're 60 or older is even worse. The odds are that you'll fail to get pregnant, or you'll miscarry, or you won't be able to raise the child. But those are just the odds. Some women can do it. More women can do it today than could have done it a century ago. Little by little, the laws of biology are changing. The fertility industry, born of that change, must change with it.